The present invention relates to an electronic flash incorporating the so-called variable guide number device or a device for controlling a flash duration.
The variable guide number device comprises in general a flash duration control circuit or a circuit for controlling the quantity of a flash of brilliant light emitted from a flash lamp, a switching circuit, an integrating circuit and a signal generating circuit for controlling the flash duration control circuit. When a shutter is released, a trigger circuit is actuated so that a flash lamp converts the energy stored on a main discharge capacitor into a flash of light. The energy charged on another capacitor is discharged through a first resistor, the flash lamp, a second resistor and a zener diode. An integrating capacitor is charged with a constant voltage across this zener diode through a variable resistor, and when the voltage across the integrating capacitor rises to a predetermined level, a first transistor is enabled so that a second transistor is enabled to apply a gate voltage to the gate of a thyristor in another trigger circuit. This trigger circuit causes a bypass lamp to conduct so that the flash lamp is turned off; that is, a flash duration is controlled.
The zener diode is therefore a power source of the variable guide number device for actuating the signal generating circuit which in turn actuates the flash duration control circuit. A desired guide number may be set with the variable resistor. In the above flash duration control system wherein the conduction timing of the first transistor is controlled by the integrating circuit consisting of the variable resistor and the integrating capacitor which is charged through the variable resistor with the voltage across the zener diode, in order to control a flash duration over a wide range from a very short flash duration to a manual flash control the current which flows from the zener diode through the variable resistor into the integrating capacitor must be varied over a wide range. Therefore the variable resistor must have a high end resistance, but because of the mechanical characteristics of the precision variable resistors and due to the manufacture tolerances, it is difficult to maintain desired accuracy or linearity tolerance over the whole range so that an error or the departure of an actual value from a setting value results. The error in the variable resistor causes an error in charging time of the integrating capacitor and hence an error in the conduction timing of the first transistor and consequently an error in flash duration control.